Good point.
To make my case, I will like to draw your attention to the fact that tax cut will benefit higher margin business more. Say we have two business, A with 3% margin, B with 6% margin. If we have 33% tax rate, then for a revenue of 100, A gives 1% to tax while B gives 2% to tax. That means B gains more margin from tax relief. If you take the case where tax is completely waived off, A’s profit after tax margin increases by 1% while B gains 2%. Even accounting for increased competition, B can still retain the excess 1%.
The argument holds for all companies having low cost competitive advantage. In our case, Dmart indeed has the higher profit margins due to its cost efficiencies, so even accounting for increased competition, it will be able to retain some of the excess gains from tax cut.